The two-component polyurethane coating materials that are in use today, based on hydroxyl-containing polyesters or polyacrylates, have to meet various requirements depending on the coated substrate.
For instance, two-component clearcoats for automotive refinish should possess excellent hardness, very good adhesion to the basecoat, outstanding solvent resistance (to super-grade petroleum, for example) and good weather stability. For these reasons, preference is given to the use of hydroxyl-containing polyacrylate resins or weather-stable polyesters whose chain molecules possess inherent rigidity.
Modern binders should additionally meet the requirements of environmental protection for high-solids and low-solvent coating materials. For this purpose use is made in particular of low molecular mass polyacrylates (See EP-A 0 638 591, EP-A 0 680 977) or polyesters (See EP-A 0 705 858), in some cases with the addition of oligomers or reactive diluents, i.e. organic compounds which are able to react with polyisocyanates as well (See EP-A 0 741 149, EP-A 0 741 158). The use of such binders in automotive refinish surfacers, i.e. highly pigmented systems, is likewise known (See EP-A 0 680 977, EP-A 0 773 247).
In the sector of automotive OEM finishing (e.g. one-component paints that are crosslinked using melamine resins), the key requirements for high-solids coating materials are different in nature. For instance, it is additionally important to achieve good mar resistance and resistance to acid rain. These properties are influenced substantially by the binder structure. Hence in this sector there is predominant use of acrylate resins having flexible monomer units and units with long linear or branched alkyl side-chains, chains, which give the coating composition inherent flexibility.
Besides the automotive sector, OH-functional acrylate resins or polyester resins are employed in general industrial coating. This embraces the large market segment of the coating of metals, plastics, and wood. While metallic substrates are coated predominantly with pigmented systems, plastic and wooden pieces are coated mainly using clearcoats. Binders are employed specifically in the sector of general industrial coating which not only have good applicability, rapid drying, good weathering stability, high gloss and fullness but also achieve a high surface quality. This means that the products should exhibit good levelling without developing the orange peel effect.
In these various application sectors it is known to blend acrylate resins and polyester resins, to produce physical mixtures or blends, in order to obtain the specific profiles and properties that are required. In this case the acrylate resins provide the gloss and weathering stability and the polyester the fullness and surface quality. However, for the producer of synthetic resins who wishes to supply the paint manufacturer with a finished, high-solids binder system it is highly complex to not only synthesize the individual components of low molecular mass but also to select and blend appropriate, compatible binders which lead to highly transparent binder systems. As a result, there has been an increase in recent times in efforts to circumvent the selection and blending of appropriate, environment-friendly binder partners by means of suitable integrated synthesis steps (See EP-A 0 776 920). Here, polyester acrylate-based binders are described that are composed of proportions by mass of from 10 to 90% polyester and from 90 to 10% acrylate resin. The polyesters are, in part, oligomers having molar masses of up to 5000 g/mol. It is precisely these polyester or ester oligomers that are of interest since they lead to very high-solids binders.
It is, however, disadvantageous that such mixed systems comprising blends of acrylate polymers with oligoesters and with corresponding polyester-modified acrylate resins with a high proportion of oligoester no longer possess the desired rapid drying (freedom from tack) and do exhibit high molecular nonuniformity and poor chemical resistance properties. The high proportion of oligoester is responsible for these negative properties.